In the operation of a copier or printer, particularly color machines, it is highly desirable to have a means for processing and enhancing graphical, text and pictorial images. Particularly in the case of single or multi-pass color printers, it is highly desirable that an image processing system be employed to meet a variety of image types or the particular gray levels in those image types. While the halftoning system of the present invention is quite suitable for use on xerographic printers in which aspects of it have been tested, it may be likewise highly useful with a variety of other xerographic as well as non-xerographic printing systems such as ink-jet or other printing techniques.
In the process of digital electrostatographic printing, an electrostatic charge pattern or latent image corresponding to an original or electronic document may be produced by a raster output scanner on an insulating medium Developing the latent image with particles of granulated material to form a powder image thereof produces a viewable record. Thereafter, the visible powder image is fused to the insulating medium, or transferred to a suitable support material and fused thereto.
Development of the latent image is achieved by bringing a developer mix into contact therewith. Typical developer mixes generally comprise dyed or colored thermoplastic particles of granulated material known in the art as toner particles, which are mixed with carrier granules, such as ferromagnetic granules. When appropriate, toner particles are mixed with carrier granules and the toner particles are charged triboelectrically to the correct polarity. As the developer mix is brought into contact with the electrostatic latent image, the toner particles adhere thereto. Non-xerographic printing systems (such as ink-jet) may be more or less successful in printing high quality images of varying types in and may involve capabilities and/or efficiencies unlike those found in xerographic systems.
Various image rendering systems have been employed to include those set forth in the following disclosures which may be relevant to various aspects of the present invention. The entire contents of all the foregoing U.S. Pat. Nos. 5,673,121; 5,740,279; 5,579,446; 5,489,991; 5,438,431; 5,394,252; 5,341,228; 5,181,162; 5,111,194; 4,595,948; 4,485,397; 4,876,611; and 4,149,194 are hereby incorporated by reference.
U.S. Pat. No. 5,673,121 discloses a method and apparatus for halftoning gray or color images by stochastic screens. An idealized stochastic screen is characterized by all of the predominant color dots (black or white) uniformly distributed. The disclosed method seeks to approach this optimization by iteratively selecting pairs of threshold levels in the screen matrix, and measuring the approach to the idealized stochastic screen. The threshold values are then swapped in position to determine whether the swap improves the measurement or not. If it does, the swap is maintained. The process is iterated until the desired result is obtained.
U.S. Pat. No. 5,740,279 discloses a method of constructing a composite halftoning screen having m times n rank ordered threshold elements. A variety of optimization techniques can be employed to improve the visual appeal of the images rendered using the composite screen for a variety of image types in single or multicolored imaging applications.
U.S. Pat. No. 5,579,446 discloses a color printer system including an interactive user interface that allows a choice between one "button" automatic control of color output or multi-button control of color output, with both automatic and manual options providing independent control for color halftoning and for color correction based on the types of objects to be printed. The preferred form allows independent print-rendering options for text, graphics and photo image.
U.S. Pat. No. 5,489,991 discloses a method for generating digital halftone matrices of a selected size for use in an image processing system in which bi-level images are formed from continuous tone input images using the digital halftone matrices. In accordance with the method, a two-dimensional array of storage positions is defined for storing values. The two-dimensional array is divided into four quadrants of equal size, each quadrant having a quadrant origin, a quadrant perimeter and four subquadrants disposed around the quadrant origin.
U.S. Pat. No. 5,438,431 discloses a method and apparatus for enabling color or black/white bi-level or multi-level halftone images to be displayed or printed on various raster output devices, in particular on display or printing devices. A dither matrix R.sup.c2n is obtained by applying on a c-fold replication of a well-dispersed threshold matrix D.sup.n a one-to-one discrete rotation of Pythagorean angle a=arctan(b/a) or .beta.=arctan(a/b) around an arbitrary point, where {a,b,c} are Pythagorean triplets satisfying Diophantine equation a.sup.2 +b.sup.2 =c.sup.2 and n is an integer. The images produced using the disclosed method are visually pleasing due to the absence of visible artifacts and to the small typical structure size.
U.S. Pat. No. 5,394,252 teaches of the advantages of stochastic or non-periodic screening over periodic screening in the suppression of moire effects.
U.S. Pat. No. 5,341,228 discloses a halftoning system using a process known as a blue noise mask. Briefly, the procedure can be described as follows: 1) Starting at one gray level with a chosen dot pattern, or "seed", the process iteratively uses a Fast Fourier Transform (FFT) techniques with a "blue noise" filter to redistribute all spots in dot pattern and eliminate large visual "clumps"; 2) Next, the dot pattern is processed at the next gray level by increasing (or decreasing) certain number of black spots on the previously determined dot pattern. Existing black (or white) spots are not moved. The same filtering technique is used to distribute newly added (or subtracted) dots; 3) Step 2 is then repeated for all gray levels sequentially. At each step, the width of the blue-noise filter varies by an amount corresponding to the current gray level; 4). The summation of dot patterns for each gray levels is the blue noise mask generated. The mask is then used to generate a halftone screen.
U.S. Pat. No. 5,181,162 discloses an object-oriented document management and production system in which documents are represented as collections of logical components, or "objects", that may be combined and physically mapped onto a page-by-page layout. Stored objects are organized, accessed and manipulated through a database management system. At a minimum, objects contain basic information-bearing constituents such as text, image, voice or graphics. Objects may also contain further data specifying appearance characteristics, relationships to other objects and access restrictions.
U.S. Pat. No. 5,111,194 discloses artificial halftone processing apparatus for binarizing input image information on the basis of the densities of pixels and combining a plurality of sets of resultant binary data to produce multi-level tone information. A matrix for halftone processing is variable in size or arrangement to store a plurality of pixel data in a memory.
U.S. Pat. No. 4,595,948 discloses superposing individual dot matrices of at least two colors. A computing unit receives electrical input signals representing the image and generates and feeds corresponding electrical signal representing the individual matrices of the at least two colors to the head means for printing. The computing means includes matrix generator means for generating the matrices in such a manner that the total number of dots to be printed in all of the matrices is limited to a first predetermined value which is less than the maximum possible total number of dots which can be printed in all of said individual dot matrices, and the total number of dots to be printed in the matrices which are formed by superposed dots of different colors is limited to a second predetermined value which is less than said first predetermined value.
U.S. Pat. No. 4,485,397 describes a method for generating a non-periodic halftone distribution by determining areas of constant or nearly constant input density and by distributing a precalculated number of print dots inside each area based on a random or pseudo random number and some spatial constraints.
U.S. Pat. No. 4,876,611 describes another stochastic screening algorithm in which the print/no-print decision is based on a recursive subdivision of the print field maintaining average density over the larger print field.
U.S. Pat. No. 4,149,194 discloses an electronic signal representing the density of a scanned image electronically halftoned by combining the pictorial signal with a rectangular array of halftone screen values to form an electronic halftone image. The halftone screen values is rotated at variable angles to the original electronic image by defining a repetitive rectangular matrix representative of any predetermined halftone cell approximating a parallelogram and incorporating a shifting parameter for the repetitive rectangular matrix.
In spite of recent advances of the various digital halftone screen designs described above with respect to the corresponding U.S. Patents, analog line screens (e.g., the analog line screen technique described in U.S. Pat. No. 4,661,859) are still widely used in currently manufactured printers. The reason for the wide use of analog line screens is that these line screens provide very high line frequencies, especially true with high addressability. Also, the analog line screens show much less noise in the image output than digital stochastic screens such as described by previously referred U.S. Pat. No. 5,673,121.
Although line screens may be preferable over stochastic screens, digital line screens are difficult to implement because there are not enough gray levels simulated by a simple line halftone screen. A simple line or cluster halftone screen has only a few elements to achieve a high line or spatial frequency, thereby reducing its achievable gray levels. For example, a line screen with five elements can generate only six different binary patterns as shown by the six steps in FIG. 4.
To address this situation others have proposed using plural halftone cells to increase the number of simulated gray-levels. An example of such a proposal is described in U.S. Pat. No. 4,903,123. However, such proposals still produce visual artificial appearance in halftone image outputs that are not necessarily of a better quality than in other digital halftone screens.
In view of these many attempts to address the situation, the present invention proposes to utilize a composite screen with stochastically distributed clusters or lines. The utilization of a composite screen produces visual effects in the image outputs which are less displeasing to the viewer.